According to the Third Generation Partnership Project (3GPP) specifications for wireless communication systems (Release 8 and later Releases), a Long Term Evolution (LTE) communication system uses orthogonal frequency division multiplex (OFDM) as a multiple access technique (called OFDMA) in the downlink (DL) from system nodes to user equipments (UEs). High-Speed Packet Access (HSPA) and early versions of LTE are sometimes called “third generation” (3G) communication systems. LTE-Advanced (Release 10 and later) has been ratified as a “fourth generation” (4G) communication system. The LTE specifications can be seen as an evolution of current wideband code division multiple access (WCDMA) specifications. The 3GPP promulgates specifications for LTE, HSPA, WCDMA, and other communication systems.
LTE communication channels are described in 3GPP Technical Specification (TS) 36.211 V9.1.0, Physical Channels and Modulation (Release 9) (December 2009) and other specifications. For example, control information exchanged by evolved NodeBs (eNodeBs) and user equipments (UEs) is conveyed by physical uplink control channels (PUCCHs) and by physical downlink control channel (PDCCHs). In an OFDMA communication system, a data stream to be transmitted is portioned among a number of narrowband subcarriers that are transmitted in parallel. In general, a physical resource block is a particular number of particular subcarriers used for a particular period of time. Different groups of subcarriers can be used at different times for different purposes and different users. OFDMA communication systems are described in the literature, for example, U.S. Patent Application Publication No. US 2008/0031368 A1 by B. Lindoff et al.
The possibility of identifying user geographical location, or position, in a system has enabled a large variety of commercial and non-commercial services, e.g., navigation assistance, social networking, location-aware advertising, emergency calls, etc. Different services can have different positioning accuracy requirements imposed by the application. In addition, some regulatory requirements on the positioning accuracy for basic emergency services exist in some countries, e.g., FCC E911 in the U.S., which puts an extra burden on the desired quality of the positioning service.
FIG. 1A illustrates a user plane of an exemplary positioning architecture in an LTE cellular communication system 100 that includes UEs 110, 120, a radio access network (RAN) that includes a plurality of eNodeBs 130-1, 130-2, . . . , 130-N, and a core network that includes a serving gateway (SGW) node 140 and a packet data network 150. The system 100 also includes a positioning node 160, which in the user plane is called a Secure user-plane Location (SUPL) Platform (SLP). In the user plane of a positioning architecture, the UEs 110, 120 are more precisely called SUPL enabled terminals (SETs).
Each eNodeB 130-1, 130-2, . . . , 130-N serves a respective geographical area that is divided into one or more cells. An eNodeB can use one or more antennas at one or more sites to transmit information into its cell(s), and different antennas can transmit respective, different pilot and other signals. Neighboring eNodeBs are coupled to each other by an X2-protocol interface that supports active-mode mobility of the UEs. An eNodeB controls various radio network functions, including for example single-cell radio resource management (RRM), such as radio access bearer setup, handover, UE uplink/downlink scheduling, etc. Multi-cell RRM functions can also use the X2-protocol interfaces. Each eNodeB also carries out the Layer-1 functions of coding, decoding, modulating, demodulating, interleaving, de-interleaving, etc., and the Layer-2 retransmission mechanisms, such as hybrid automatic repeat request (HARQ). The eNodeBs 130-1, 130-2, . . . , 130-N are coupled to one or more SGWs 140 (only one of which is shown in FIG. 1A).
FIG. 1B illustrates a control plane of the exemplary positioning architecture in the LTE communication system 100. In the control plane as shown, an LTE-Uu protocol interface couples the UE 110 to the eNodeB 130, and an S1-MME protocol interface couples the eNodeB 130 to a Mobility Management Entity (MME) 140, which is a name for the SGW in the control plane. The positioning node 160 is called an evolved Serving Mobile Location Center (E-SMLC) in the control plane, and is coupled to the MME 140 by a signaling link selection (SLs) protocol interface. It will be understood that there can be a communication interface between the SLP and E-SMLC for interworking in the positioning node 160. 3GPP has standardized two protocols specifically to support positioning in LTE: an LTE Positioning Protocol (LPP) and an LTE Positioning Protocol Annex (LPPa). Messaging according to those protocols is also depicted in FIG. 1B.
The LPP is a point-to-point protocol between a location services (LCS) server, such as the E-SMLC 160, and a LCS target device, such as the UE 110, that is used to position the target device. Transmitted LPP messages are transparent to an MME 140, and use radio resource control (RRC) protocol messages for transport over an LTE-Uu interface between the UE 110 and the eNodeB 130, and then S1 application protocol (S1AP) messages over the S1-MME interface between the eNodeB 130 and the MME 140, and then LCS-AP messages over the SLs interface between the MME 140 and the E-SMLC 160. LPP is defined in 3GPP TS 36.355 V9.2.1, LTE Positioning Protocol (LPP) (Release 9) (June 2010), for example.
LPPa is a protocol for an interface between an eNodeB and a positioning server, such as the E-SMLC 160. LPPa messages are also transparent to the MME 140, which routes LPPa message packets over the S1-MME and SLs interfaces without knowledge of the involved LPPa transactions. LPPa is specified only for control-plane positioning procedures, but with user plane/control plane interworking, LPPa can also assist the user plane by querying eNodeBs for information and eNodeB measurements not related to a UE connection. LPPa is defined in 3GPP TS 36.455 V9.2.0, LTE Positioning Protocol A (LPPa) (Release 9) (June 2010), for example.
In the user-plane positioning architecture, the SUPL service uses established data-bearing channels (i.e., the LTE user plane) and positioning protocols (i.e., LPP) for exchanging the positioning-related data between a LCS target (e.g., a SET 110, 120) and a LCS server (e.g., a SLP 160).
UEs 110, 120 are generally wireless communication devices that can be cellular radiotelephones, personal digital assistants (PDAs), Personal Communications System (PCS) terminals, laptop computers, palmtop computers, or any other type of device or appliance that includes a communication transceiver that permits the device to communicate with other devices via a wireless link. A PCS terminal can combine a cellular radiotelephone with data processing, and facsimile and data communication capabilities. A PDA can include a radiotelephone, a pager, an Internet/intranet access device, a web browser, an organizer, calendars, and/or a global positioning system (GPS) receiver. One or more of UEs 110, 120 can be referred to as a “pervasive computing” device. In some implementations, the UEs 110, 120 can include wireline telephones (e.g., Plain Old Telephone system (POTs) telephones) that are connected to a Public Switched Telephone Network (PSTN). In a positioning architecture like that depicted in FIG. 1A, a UE can also be a base station, signal relay, radio repeater, sensor, etc.
As described in 3GPP TS 36.305 V9.3.0, Stage 2 Functional Specification of User Equipment (UE) Positioning in E-UTRAN (Release 9) (June 2010), for example, the positioning node 160 can determine the geographic positions of UEs in the system 100 in a wide variety of ways, e.g., Global Navigation Satellite System (GNSS), Observed Time Difference Of Arrival (OTDOA), Uplink Time Difference Of Arrival (UTDOA), Enhanced Cell ID (E-CID), radio fingerprinting, etc. GNSS is a generic name for satellite-based positioning systems with global coverage. Examples of GNSS systems include the U.S. GPS, the European Galileo, the Russian Glonass, and the Chinese Compass. With GNSS, a position is typically obtained by triangulation based on measurements of times of arrival of reference signals. OTDOA uses timing measurements conducted on downlink (DL) reference signals received from multiple locations, and UTDOA uses timing measurements performed on UL reference signals received at multiple locations.
In OTDOA and UTDOA, the position is obtained by multi-lateration or triangulation based on intersections of hyperbolas or circles. Methods based on multi-lateration, which is a way to determine a geometrical position from intersection of multiple surfaces, e.g., spheres or hyperboloids, require measurements from multiple sites, such as eNodeB antennas, with a good geometry; ideally at least three such sites are necessary for a two-dimensional (2D) position and four sites for a three dimensional (3D) position, which in practice means that a UE needs to measure significantly more cells, also because some of them are co-located.
In radio fingerprinting positioning, the positioning node 160 uses information in a radio fingerprint database that stores radio fingerprints derived from Evolved Universal Terrestrial Radio Access Network (E-UTRAN) and/or Inter-Radio Access Technology (IRAT) measurement data. The E-UTRAN and/or IRAT measurement data can be provided to the positioning node 160 in conjunction with precise geographic position data obtained at the same geographic location at which the E-UTRAN and/or IRAT measurements were performed (e.g., GPS position data). The positioning node 160 can subsequently receive E-UTRAN and/or IRAT radio fingerprint measurement data from UEs 110, 120 and perform a lookup in the radio fingerprint database to identify a stored radio fingerprint that matches the received E-UTRAN and/or IRAT radio fingerprint measurement data, and to retrieve a stored geographic position that corresponds to the matching radio fingerprint. The positioning node 160 can provide this geographic position to the UE that sent the radio fingerprint measurement data, or to other destinations, such as, for example, an emergency or police call center.
The network 100 can exchange information with one or more other networks of any type, including a local area network (LAN); a wide area network (WAN); a metropolitan area network; a telephone network, such as a public switched terminal network or a public land mobile network; a satellite network; an intranet; the Internet; or a combination of networks. It will be appreciated that the number of nodes illustrated in FIG. 1 is purely exemplary. Other configurations with more, fewer, or a different arrangement of nodes can be implemented. Moreover, one or more nodes in FIG. 1 can perform one or more of the tasks described as being performed by one or more other nodes in FIG. 1. For example, parts of the functionality of the eNodeBs can be divided among one or more base stations and one or more radio network controllers, and other functionalities can be moved to other nodes in the network.
FIG. 2 is a frequency-vs.-time plot showing an arrangement of downlink (DL) subcarriers, or tones, in an LTE system. In general as specified in 3GPP 36.211, DL signals in the frequency division duplex (FDD) mode of LTE are organized into successive frames of 10 milliseconds (ms) duration. Each frame is divided into ten successive subframes, and each subframe is divided into two successive time slots of 0.5 ms. Each slot includes either three, six or seven OFDM symbols, depending on whether the symbols include long (extended) or short (normal) cyclic prefixes. An LTE physical resource block (RB) comprises a group of resource elements (REs) spanning twelve consecutive subcarriers in the frequency domain and one time slot in the time domain. A physical RB is illustrated by the shaded area in FIG. 2 for symbols having a normal cyclic prefix. The subcarriers are spaced apart by fifteen kilohertz (kHz) and together occupy approximately 180 kHz in frequency. In an Evolved Multicast Broadcast Multimedia Services (MBMS) Single Frequency Network (MBSFN), the subcarriers are spaced apart by either 15 kHz or 7.5 kHz. A RE spans one subcarrier (frequency domain) and one symbol (time domain). It will be understood that RBs could include other numbers of subcarriers for other periods of time in other communication systems.
In the case of OFDM transmission, an eNodeB transmits reference signals comprising known reference symbols on known subcarriers in the OFDM frequency-vs.-time grid. For example, cell-specific reference signals (CRS) are described in Clauses 6.10 and 6.11 of 3GPP TS 36.211 V9.0.0, Evolved Universal Terrestrial Radio Access (E-UTRA), Physical Channels and Modulation (Release 9) (December 2009). A UE uses its received versions of the known reference signals to estimate characteristics, such as the impulse response, of its DL channel. The UE can then use the estimated channel matrix for coherent demodulation of the received DL signal, and obtain the potential beam-forming gain, spatial diversity gain, and spatial multiplexing gain available with multiple antennas. In addition, the reference signals can be used to do channel quality measurement to support link adaptation.
Up to four CRS corresponding to up to four transmit antennas of an eNodeB are currently specified, and FIG. 3A shows the arrangement of reference symbols in a subframe for one antenna, FIG. 3B shows the arrangement of reference symbols in a subframe for two antennas, and FIGS. 3C, 3D depict the arrangement of reference symbols in a subframe for four antennas.
FIG. 3A shows a frequency-vs.-time grid that includes reference symbols R0 that are transmitted at known subcarrier and time symbols in a subframe from an eNodeB having one antenna port 0. In FIG. 3A, the reference symbol R0 is depicted as transmitted on every sixth subcarrier in OFDM symbol 0 and OFDM symbol 4 in every seven-symbol (normal cyclic prefix) time slot. Also, the reference symbols R0 in OFDM symbol 4 are offset by three subcarriers relative to the reference symbols in OFDM symbol 0, the first OFDM symbol in a slot. It should be understood that the reference symbols R0 can be transmitted in other OFDM symbols depending on whether the symbols have long or short cyclic prefixes. For example, the reference symbols R0 can be transmitted in OFDM symbol 3 when the OFDM symbols have long cyclic prefixes.
FIG. 3B shows frequency-vs.-time grids that include reference symbols R0 that are transmitted at known frequencies and time instants in a subframe from an antenna port 0 (which is the same as FIG. 3A) and reference symbols R1 that are transmitted at known frequencies and time instants in a subframe from an antenna port 1. Cross-hatched REs indicate reference symbols that are not transmitted by a particular antenna port.
FIGS. 3C and 3D show frequency-vs.-time grids that include reference symbols R0 from an antenna port 0 (which is the same as FIG. 3A), reference symbols R1 that are transmitted from an antenna port 1 (which is the same as FIG. 3B), reference symbols R2 that are transmitted from an antenna port 2, and reference symbols R3 that are transmitted from an antenna port 3. As in FIG. 3B, cross-hatched REs in FIGS. 3C, 3D indicate reference symbols that are not transmitted by a particular antenna port. It will be noted in FIG. 3D that the reference symbols R2, R3 are depicted as transmitted in OFDM symbols 1, 5, respectively, in every seven-symbol time slot.
Some communication systems, such as LTE-Advanced, can employ more than four transmit antennas in order to achieve better performance. For example, a system having eNodeBs with eight transmit antennas will need extensions of the LTE CRS signals described above.
To enable positioning in LTE and facilitate positioning measurements of a proper quality and for a sufficient number of distinct locations, new physical signals dedicated for positioning, called positioning reference signals (PRS), have been introduced and specific positioning subframes have been agreed in 3GPP, although the existing CRS described above can in principle also be used for positioning.
PRS and Positioning Subframes in LTE
PRS are transmitted from one antenna port (R6) according to a pre-defined pattern, as described for example in Clause 6.10.4 of 3GPP TS 36.211 V9.0.0, Evolved Universal Terrestrial Radio Access (E-UTRA), Physical Channels and Modulation (Release 9) (December 2009). One of the currently agreed PRS patterns is shown in FIG. 4, which corresponds to the left-hand side of FIG. 6.10.4.2-1 of 3GPP TS 36.211, where the grey squares indicate PRS resource elements that include reference symbols R6 from an antenna port 6 within an RB in frequency and one subframe in time with the normal cyclic prefix. A physical broadcast channel (PBCH) can be transmitted from one or two antenna ports in the RBs.
A set of frequency shifts can be applied to the pre-defined PRS patterns to obtain a set of orthogonal patterns which can be used in neighbor cells to reduce interference on the PRS and thus improve positioning measurements. The effective frequency reuse of six can be modeled in this way. The frequency shift is defined as a function of Physical Cell ID (PCI) as follows:vshift=mod(PCI,6)in which vshift is the frequency shift, mod( ) is the modulo function, and PCI is the Physical Cell ID. The PRS can also be transmitted with zero power, or muted.
To improve hearability of the PRS, i.e., to enable a UE to detect the PRS from multiple sites and with a reasonable quality, positioning subframes have been designed as low-interference subframes, i.e., it has also been agreed that no data transmissions are allowed in general in positioning subframes, although a network can at its own risk still allow some DL transmission in positioning subframes. As a result, synchronous networks' PRS are ideally interfered with only by PRS from other cells having the same PRS pattern index, i.e., the same frequency (vertical) shift (vshift), and not by data transmissions.
In partially aligned asynchronous networks, PRS can still be interfered with by transmissions over data channels, control channels, and any physical signals when positioning subframes collide with normal subframes, although the interference is reduced by the partial alignment, i.e., by aligning the beginnings of positioning subframes in multiple cells within one-half of a subframe with respect to some time base. PRS are transmitted in pre-defined positioning subframes grouped by several consecutive subframes (NPRS), i.e., one positioning occasion, which occur periodically with a certain periodicity of N subframes, i.e., the time interval between two positioning occasions. The currently agreed periods N are 160, 320, 640, and 1280 ms, and the number of consecutive subframes NPRS can be 1, 2, 4, or 6, as described in 3GPP TS 36.211 cited above.
With the cell PCI, the PRS configuration (comprising the offset from system frame number (SFN) 0, periodicity, and the number of positioning subframes) is signaled to the UE as a part of the OTDOA assistance data from the positioning server (e.g., an E-SMLC) to a positioning target (e.g., a UE) using the LPP protocol. The PRS pattern for PRS resource elements in the time-frequency domain, as described above, can be found out by the UE from the cell PCI.
Correlation Properties of Reference Signal Patterns
Signals, including reference signals used for positioning (PRS or CRS in LTE), typically do not have ideal correlation (auto- and cross-correlation) properties. Better auto-correlation properties enhance the resolvability of multipath, which is very important, for example, in urban environments where OTDOA is expected to complement Assisted Global Positioning System (A-GPS). Poor auto-correlation properties may also affect the search window size, which is used to identify the PRS pattern by detecting the correlation peak. For instance, poor auto-correlation properties of the PRS pattern constrain the UE to use a more precise search window in order to avoid searching for the unnecessary correlation peaks (i.e., side lobes). On the other hand, the search window depends on the UE location uncertainty. Ideally, the maximum search window is defined by a range [−r, r], where r is the maximum cell range.
A general observation known in the art is that the best auto-correlation properties are achieved when the signal is transmitted over all subcarriers (with uniform sum energy density over the subcarriers) during a coherent time interval, although not necessarily on all subcarriers in each OFDM symbol. This is because the auto-correlation of a periodic function is, itself, periodic with the same period, which means that the presence of a periodic component in the pattern may generate side-lobe auto-correlation peaks.
Transmitting the signal over the entire bandwidth in a symbol is not a good approach in a synchronous network from an interference-management point of view, unless predefined time-offsets are applied in different cells to mimic frequency reuse. This means that it is preferable to transmit reference signals according to a predefined pattern. Furthermore, a higher frequency reuse is desirable in networks where the interference is crucial (e.g., with high load, short inter-site distance, etc.). Such a sparseness property is enjoyed, for example, by patterns designed based on Costas arrays, which are traditionally used in sonar and radar communication. A Costas array is a geometrical set of n points lying on the squares of a n×n checkerboard, such that each row or column contains only one point, and that all of the n(n−1)/2 displacement vectors between each pair of dots are distinct. In practice, however, it is not always possible to achieve patterns with optimal correlation properties.
The frequency-time transmission patterns for PRS currently defined by 3GPP TS 36.211 have correlation properties that can be insufficiently good for positioning in a “rich” multipath environment, i.e., an environment with a large number of multipath signals. As an illustrative example, FIGS. 5A, 5B show the correlator outputs for twenty-five PRS RBs with normal and extended CP, respectively, versus time shown in meters (converted by multiplying time by the speed of light in meters per second). Periodic strong side lobes can be seen, for example, at about 3 km, 6 km, and so on.
Furthermore, there is no possibility to control PRS power within a subframe, because, as stated in Clause 5.2 of 3GPP TS 36.213 V9.3.0, Evolved Universal Terrestrial Radio Access (E-UTRA), Physical Layer Procedures (Release 9) (September 2010): “A UE may assume that downlink positioning reference signal EPRE [energy per resource element] is constant across the positioning reference signal bandwidth and across all OFDM symbols that contain positioning reference signals in a given positioning reference signal occasion.”